Coincidentally or otherwise, that's something I've just done for our test bay. We have a utility company transformer in our yard and, like others here, It proved very difficult to get information from them. Eventually I got the Z and estimated x and R from that.Is this study for short circuit ratings on the equipment or arc flash.
If for short circuit, try to find the size of the closest upstream transformer from your service and use the infinite primary calculated through to the secondary. 8 is what I use if I have to take a guess for x/r. That will give you a conservative number to use.
If you are calculating for arc flash, you will need to be creative and make that short circuit available value lower.